1. Field of the Invention
The present invention relates to a silver halide photographic emulsion and, more particularly, to a photographic emulsion providing an extremely contrasty negative image photographic property. Further, the present invention relates to a process for forming images and, more particularly, to a process for forming photographic images with an extremely contrasty negative image.
2. Description of the Prior Art
U.S. Pat. No. 2,419,975 describes a process for obtaining a contrasty negative image photographic property by adding a hydrazine compound. The disclosure in this U.S. patent is that an extremely contrasty photographic property of a gamma(.gamma.) of more than 10 can be obtained by adding a hydrazine compound to a silver chlorobromide emulsion and developing the emulsion with a developer having a pH as high as 12.8. However, a strongly alkaline developer whose pH is near 13 tends to be oxidized by air and is so unstable that it cannot be stored or used for a long time.
A super-contrasty photographic property of a gamma of more than 10 is extremely useful for photographic reproduction of continuous tone images through dot images which are useful for making printing plates regardless of whether the image is negative or positive, or useful for reproduction of line images. For such a purpose, the process comprising using a silver chlorobromide photographic emulsion containing more than about 50 mol%, preferably more than 75 mol%, of silver chloride and developing the emulsion with a hydroquinone developer wherein the effective concentration of sulfite ion is controlled to an extremely low level (usually not more than about 0.1 mol/l) has generally been employed. However, this process has the defect that, since the sulfite ion concentration in the developer is low, the developer is extremely unstable and cannot be stored for longer than about 3 days. In addition, a high sensitivity cannot be obtained with a silver chlorobromide emulsion containing a slight amount of silver bromide. Therefore, it has strongly been desired to obtain a super-contrasty photographic property useful for the reproduction of dot images or line images using a highly sensitive emulsion and a stable developer.